The present invention is a mechanism incorporated in each of the blades of one or more rotor stages of a compressor or turbine of a gas turbine engine. The blades of the rotor are circumferentially distributed on a disk for rotation therewith about the disk axis. A conventional rotor blade has a root or dovetail portion which is slidably received in a complementarily configured recess provided in the rotor disk, a platform portion located outside the rotor disk, an airfoil portion extending radially outwardly from the platform and a shroud (sometimes segmented) located at the tips of the airfoils, each shroud segment being connected to a corresponding blade tip.
The platforms collectively define a radially outwardly facing wall and the tip shroud segments collectively define a radially inwardly facing wall of an annular gas flow passageway through the engine. The airfoils of the rotor blades extend radially into the passageway to interact aerodynamically with the gas flow therethrough.
These airfoils are subject to fatigue due to vibrations even though the angular speeds are low, for example, 4000 rpm. It is necessary to damp such vibrations to reduce the fatigue on the blades, particularly at or near resonant frequencies.
Various types of blade dampers are known. For example, one type of damper consists of so-called blade-to-ground dampers which generally have a movable member positioned between the coverplate assembly and the underside of the platform of one or more turbine blades. Upon rotation of the turbine, the member is displaced radially outwardly by centrifugal forces to engage the coverplate and underside of a blade platform, thereby perform a damping function. A damper of this type is disclosed in U.S. Pat. No. 4,355,957 to Sifford et al.
Other vibration dampers perform blade-to-blade damping. In one kind of blade-to-blade damper, the damping member is displaced radially outwardly by centrifugal forces during engine operation to engage the undersides of the platforms of adjacent rotor blades. Blade-to-blade dampers of this type are disclosed in U.S. Pat. No. 4,182,598 to Nelson, U.S. Pat. No. 4,347,040 to Jones et al., U.S. Pat. No. 4,568,247 to Jones et al., U.S. Pat. No. 4,872,810 to Brown et al. and U.S. Pat. No. 4,917,574 to Dodd et al.
In the Nelson, Brown and Dodd patents, the dampers are trapped between the rotor disk and the underside of the platform of one or more turbine blades. In the Jones patents, each damper is loosely retained by a rivet which allows radial and circumferential movements.
In yet another type of blade-to-blade damper, a member is placed in a pocket between the platforms of adjacent blades. U.S. Pat. No. 2,912,223 to Hull teaches a spring-like member arranged between the platforms of adjacent blades to damp vibrations of the blades and seal the gap between adjacent platforms. U.S. Pat. No. 4,497,611 to Keller teaches an axial flow turbine wherein a wedge-shaped member is axially displaced by an axial pressure difference, whereby turbine blade vibration is damped.
In accordance with the teaching of U.S. Pat. No. 4,872,812 to Hendley et al., vibration damping and gap sealing are provided by elongated inserts of equilateral triangular cross section loosely received within pockets at the spacing gaps between opposing convex and concave airfoil side edges of adjacent blade platforms. The inserts and pockets are relatively configured to provide self-orientation of the inserts into gap sealing and vibration damping engagement with the platform edges when acted upon by centrifugal forces due to rotor rotation.
An improved vibration damper for a turbine rotor blade was disclosed in U.S. Pat. No. 4,936,749 to Arrao et al. A U-shaped wire-form damping member has its legs respectively slidably received in a pair of inclined recesses which extend into the platform portion of the blade and toward the root portion.
Another blade-to-blade vibration damper for rotor blades of a gas turbine engine is disclosed in copending U.S. patent application Ser. No. 07/770,839 to Steckle et al., entitled "Turbine Blade Platform Damper", now U.S. Pat. No. 5,215,442, which application is commonly assigned to the assignee of the present application. That improved vibration damper comprises a cylindrical shaft and a disk-like contact head connected thereto. The shaft is slidably inserted in a cylindrical recess incorporated in the rotor blade platform.
Finally, U.S. Pat. No. 1,554,614 to Allen discloses a wedge-shaped damper interposed between shroud elements and retained by spring elements carried on the inner sides of the shroud elements.